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Battery Desulfator

Hi all,
I want to build a desulfator from a 50V moded ATX PS.
When I power the PS through a light bulbe it works but when I plug it directly in the 220V it don't work !!!
I use a current transformer to sense the current in the drain of the mosfet.
Of course when powered through the light bulbe it lights so no need to CT.
Any ideas please?

You cannot just switch 50volts in a 12v battery you have to control the voltage as well. What is probably happening is that your smps is tripping on short circuit protection. You can also try larger inductance on the output of the smps.

1- your smps is tripping on short circuit protection.
2- I am assuming that you are talking of a battery charger (Desulfator)

Thanks Silvio for your help.
1- Yes, the PS shuts down (short circuit protection). I tried a NTC (like the one in the input of the smps) in series with the BAT, to limit the current at startup, but with no success !!
2- I'm talking about a desulfator (regenerator), in this case it's equivalent to a pulse charger.

The frequency of pulses is about 1KHz and the width about 100us.
How to disable the short circuit protection for pulses under 120us?

You can make variable voltage and current and you have a decent battery charger. KA7500 is the same as TL494. In the link provided you will find also a battery charger made of ATX power supply, This works I tried it.

Many thanks dear Silivio again, but I don't need a charger I'm looking for a desulfator to regenerate a sulfated battery.
There are many circuits on the web but they use at least one 200 uH bobine. I hate bobines !!!

Oh I see Delta 67 I taught it was a charger in another language. Well as you can see from your result the SMPS is tripping on overload. A choke in the Drain will limit current to a safe level. I have taken a look at the disulfator from Zero fossil fuel on youtube and I could also see that he is using a P channel mosfet instead. He is also using inductors. At times too simple circuits do not work as they should. If you want to build something see that who published it tried it out and works. Inductors can be found in many old smps from old computers. See that the pulses from the 555 timer are at minimum duty cycle on start up otherwise you will face the same problem again. Did you try to check the additional circuit you added first and see what is happening on an oscilloscope?

The additional 555 circuit works as a charm. The frequency of pulses is about 1KHz and the width about 100us.
There's a choke in the circuit (the original one for 12V rail), , it's not schown on the shematic, I don't know it's value.

A current limit has to be applied somewhere, the pulse duration however small is too long compared with the frequency of the smps which is around 20Khz, You can try an inductor with the drain or source of the fet so that it will limit the current and during the off cycle it will discharge back to the battery. Otherwise disable the short circuit protection on the smps but the result of that is unknown. it will most likely blow. An inductor however should work for you.